Problems in realizing a digital vocoder and novel solutions

Some details are given of a channel vocoder intended for use as a speech output device for data-processing systems, but also suitable for voice transmission with 2400 bit/s, e.g., in narrow-band digital radio systems or in digital encryption systems. Characteristic features are, in the analyzer, the use of switched op-amp integrators instead of conventional low-pass filters plus rectifiers, and digital amplitude compression; in the synthesizer, pulse excitation which, in the case of unvoiced sounds, is achieved by means of a pseudorandom pulse derived from a digital shift-register circuit. This pseudorandom pulse exhibits very good stability and reproducibility.